Metalloid Aluminum Clusters with Fluorine

Metals have a very high energy density compared to explosives, but typically release this energy slowly via diffusion-limited combustion. There is recent interest in using molecular-scale metalloid clusters as a way to achieve very rapid rates of metal combustion. These clusters contain a mixture of low-valence metals as well as organic ligands. Here we investigate a prototypical aluminum metalloid cluster to determine system stability if the organic ligand contains significant amounts of fluorine. The fluorine can, in principle, oxidize the metallic elements, resulting in a system much like organic explosives where the fuel and oxidizer components are mere angstroms apart. We performed density functional theory calculations within the SIESTA code to examine the cluster binding energy and electronic structure. Partial fluorine substitution in a prototypical aluminum-cyclopentadienyl cluster results in increased binding and stability, likely due to weak non-covalent interactions between ligands. Ab initio molecular dynamics simulations confirm that the cluster is structurally stable when subjected to simulated annealing at elevated temperatures.